Several patents or papers have been reported on filamentous nano-carbon (carbon nanofiber or graphite nanofiber) and its preparation. For examples, Exxon Research and Engineering Co. (USA) disclosed production of carbon filaments by dissociating a carbon-containing gas such as carbon monoxide, acetylene, etc. at a temperature to about 800° C. in the presence of iron monoxide or iron (U.S. Pat. No. 4,565,683). Also, Hyperion Catalytic International Inc. (USA) disclosed the multi-walled carbon nanotube, which is characterized by a cylindrical shape, a hollow core, the aspect ratio of more than 5, an ordered outer region of multiple, and substantially continuous 8˜15 layers of ordered carbon atoms having an outside diameter between about 10 and 15 nanometers which are catalytically grown from a gaseous carbon-containing compound (Japan Patent No. 62-5000943).
Baker and Rodriguez (U.S. Pat. No. 6,099,960; High surface area nanofibers, methods of making, methods of using and products containing same) disclosed preparation of carbon nanofibers with 50˜800 m2/g surface areas through catalytic pyrolysis of several hydrocarbons over catalysts such as iron, nickel, and cobalt at 500˜700° C. Boehm (Boehm, Carbon, 11, 583 (1973)), Murayama (H. Murayama and T. Maeda, Nature, 245, 791), and Rodriguez (N. M. Rodriguez, J. Mater. Res. 8: 3233 (1993)) reported preparation of filamentous nanocarbons or carbon nanofibers through catalytic pyrolysis over alloys of iron, cobalt, and nickel.
Since report on carbon nanotube and its preparation (S. Iijima, Nature, 354, 56 (1991)), a number of studies on preparation and application of carbon nanofiber or filamentous nanocarbons have been performed in the last decade. Carbon nanotube is a nano-sized fibrous carbon of cylindrical shape with a hollow core of more than 0.4 nm, wherein the hexagonal planes align parallel to the fiber axis. Carbon nanotube is classified as multi-walled carbon nanotube (MWCNT, concentric stacking of multi layers) and single walled carbon nanotube (SWCNT, only one layer). SWCNT has 0.4˜5 nm diameters, and the outer diameter of MWCNT ranges 2.5˜50 nm.
Comparing to the carbon nanotube, filamentous nanocarbons or carbon nanofibers have the carbon hexagonal planes stacking perpendicular to the fiber axis (columnar or platelet structure, see FIG. 7) and angled to the fiber axis (herring bone or feather structure, see FIG. 8, ref.) Rodriguez, N. M. 1993. J. Mater. Res. 8: 3233). Carbon nanofibers have no continuous hollow core, differing from carbon nanotube. Such carbon nanofibers have been synthesized by catalytic pyrolysis of hydrocarbons or carbon monoxide over VIII metals such as Fe, Co, and Ni as main catalysts.
Carbon nanofibers or filamentous nanocarbons in practical applications have attracted attention no more than as a substitute for carbon black, whereas carbon nanotubes, which are characterized by the diameter of several or several tens nanometers, are expected for many applications: for examples, conductive pigments or composites especially with transparency; the field emission; nano-electronics; hydrogen storage; and biotech-relating applications.
Such a low potential of carbon nanofibers may be originated from relatively large diameters of more than 100 nm, actually further 300 nm in many fibrous carbons, for which the transparency to visible light can be never expected in solvent or composite containing even less than 1 wt % of them; and for which the conductivity in composites is inferior to carbon blacks due to inferior contact property. Generally, the transparency to visible light can be attained when the particle size or diameter is controlled below 100 nm, preferentially 80 nm. Carbon nanofibers so far have a wide distribution of diameters, furthermore thicker average diameters than 100 nm as above-mentioned. Hence, many advantages arising from a nano-size such as transparency cannot be expected, and it is difficult to homogeneously control the properties in their practical applications.
Although there are many problems relating to carbon nanofiber and its application as aforementioned, carbon nanofibers or filamentous nanocarbons are characterized by the superior productivity, which is several or several tens times higher than that of carbon nanotube, depending on preparation methods. Such high yield results in low prices. Also, superior properties of carbon nanofibers have been reported in some applications, especially hydrogen storage: for examples, Baker and Rodriguez reported a marvelous result of 40˜63 wt % hydrogen storage (U.S. Pat. No. 6,159,538). Although such a surprising result has been proved not to be reproducible (USA DOE Report, IEA Task 12: Metal Hydride and carbon for Hydrogen Storage 2001, Project No. C-3-Leader: Richard Chahine (Canada), Assessment of Hydrogen Storage on Different Carbons), the same report or other papers suggests that carbon nanofibers are capable of hydrogen storage about 2 times more than active carbons under 10 MPa (R. Stroebel, et al., J. Power Sources, 84, 221 (1999)).
Moreover, carbon nanofibers or filamentous nanocarbons produced over non-supported catalysts are advantageous in terms of prices, as they never need burdensome and high cost purification.